1. Field of the Invention
The present invention relates to a composite superconductive body, for use in diamagnetic levitation systems, and a superconduction magnetic levitation system for levitating and driving a superconductive levitation body. The present invention may be applied for example to a carrier system for articles.
2. Description of the Prior Art
Rapid and dust-free carrying are available in a non-contact carrier because no friction is caused at a running surface. A magnetic levitation type linear motor is one example of a non-contact carriers. A magnetic repulsion type magnetic levitation train, which is known, has been developed with a principal object of high speed transport of passengers, and uses superconductive magnets because a high magnetic field is requested so as to support the weight of the train and passengers.
In a hospital or semiconductor factory where dust-free transport is required, a magnetic repulsion type or magnetic attraction type magnetic levitation linear motor is used. In dust-free transport where carrying of heavy articles is not required, normal conductive magnets are used. As mentioned above, various magnetic levitation linear motors are available depending on application. In every motor, however, magnetic field control for supporting levitation is difficult and the carrier is expensive.
Following the discovery of high temperature superconductors, i.e. superconductors having a critical temperature higher than the temperature of liquid nitrogen (77K), magnetic levitation using the diamagnetism of a superconductor has attracted attention. Magnetic levitation and driving using the diamagnetism can be realized by a comparatively simple structure of a combination of magnets and superconductors and complicated magnetic field control for supporting levitation is not required.
In the magnetic levitation train mentioned above using superconductive magnets, a large current flows through the train because the electric resistance of a superconductor is zero and a high magnetic field is generated on the side of the body of the train, and hence the magnetic effect on the passenger space comes into question.
By making an induced current flow in each levitation coil installed on the ground, magnetic repulsion force is generated between the levitation coil and the superconductive magnet of the body so as to obtain levitation force. Therefore, when the train is stopped, the magnetic field in the levitation coil does not change, no induced current flows, and the levitation force is zero.
In the case of low speed running, the magnetic field changes little and sufficient levitation force also cannot be obtained. Therefore, when the train is stopped or runs at a low speed, the body is supported by the wheels, and it is impossible that the body is always kept levitated.
In the case of a magnetic levitation train using the diamagnetism instead of the magnetic levitation train described above, the levitation force is obtained even when the train is stopped and the magnetic repulsion force arises by shielding the magnetism. This is known as the Meissner effect.
Therefore, by using superconductors on the levitation body, the magnetic effect in the train of the magnets on the ground can be reduced. By magnetic levitation and driving using diamagnetism, inexpensive and dust-free non-contact carrying with sophisticated performance can be realized, unlike a carrier using a conventional magnetic levitation linear motor.
Previously proposed diamagnetism type magnetic levitation devices using high temperature superconductors will now be described.
(1) In Nikkan Kogyo Shimbun issued on Nov. 25, 1987, it is described that high temperature superconductors are installed along a slope so as to form a track and levitated magnets run on the sloping track using the gravity as a driving source.
(2) As described in Yomiuri Shimbun issued on Mar. 28, 1989, the high temperature superconductors and magnets of (1) above are reversely arranged and the superconductors are used as a levitation body. As driving force, gravity is used by means of a slope in the same way as in (1).
(3) As described in Lecture Journal of 12th Meeting of Japanese Applied Magnetics Society, Sep. 30, 1988, p. 18, superconductors levitated on magnets move horizontally by following magnets on the ground which are moved mechanically.
(4) It is described in Lecture Journal of Autumn Low Temperature Engineering, 1988, p. 137, that magnets levitated by split stripe-shaped superconductors on the ground are moved horizontally by controlling the superconducting status of each superconductor. The transition between superconductivity and normal conductivity is used as driving force.
(5) In the method proposed in Japanese Patent Application Laid-Open No. 2-250305 (Application No. 1-70634), a track having a uniform magnetic field in the running direction is constructed by levitation magnets and superconductors levitated on the track are moved horizontally using the changing magnetic field of driving magnets as a magnetic field slope.
(6) An unpublished proposal with supplements (5), relates to an induction type linear motor whereon the arrangement of levitation magnets for obtaining lateral guidance force during running and the most suitable shape and arrangement of driving coils are specified (Japanese Patent Application No. 2-288536 and corresponding U.S.A. and European patent applications, none of them yet published).
(7) A synchronizing linear motor for moving magnets levitated on a track constructed by high temperature superconductors by exciting coils on the ground by a three phase alternating current is described in Lecture Journal of Spring Low Temperature Engineering, 1990, p. 110.
In the above prior proposals, the driving force in (1) and (2) is derived from the slope and the control of driving is not taken into consideration. In (3), magnets are required to be moved mechanically and a mechanically sliding section exists. Therefore, this is not suited to non-contact carrying which aims at clean carrying.
In (4), the running speed of the levitation body is affected by the transition speed between superconductivity and normal conductivity and the control is very difficult. Furthermore, there is a problem that a high driving force cannot be obtained.
In (5), since a magnetic field slope is used as a driving source, there are problems arising that a high driving force cannot easily be obtained and the driving efficiency and the controllability are not so good. Furthermore, the lateral guidance force during running is not taken into sufficient consideration. In the above proposals therefore, there is a problem that the controllability for a levitation train or carrier is not sufficient.
In (6), to complement (5), the controllability is enhanced by increasing the guidance force during running by providing a magnetic field distribution so that the field is uniform in the running direction and a magnetic field wall is obtained in the lateral direction and by optimizing the driving coils.
However, enlargement of the levitation body or superconductors in scale-up of the device is not taken into special consideration. Since high temperature superconductors which can be used by liquid nitrogen temperature have ceramic fragility, they are lacking in workability. Enlargement by mechanical connection is difficult. Manufacture of large-scale integrated superconductors is difficult due to occurrence of cracks or restrictions of the manufacturing process.
In (7), small pieces of high temperature superconductors are laid on the ground for scale-up of the track. However, the superconductor status of the scaled-up track is required to be maintained and a large scale cooling device is required. In this device, the levitation body (magnet) has magnetic poles and the magnetic poles move in synchronization with the proceeding magnetic field generated by exciting the driving coils by a three phase alternating current.
In such a synchronizing linear motor, the accuracy of the pitch between driving coils is important. Assuming that the magnet length in the running direction is L and the pitch is p, the following equation is required to hold: EQU p=(2/3)L
When the equation does not hold, the attraction and repulsion between the driving coils are unbalanced and the levitation body moves (vibrates) up and down. In an extreme case, the levitation body comes in contact with the track on the ground. When scaling up the track and subjecting the driving coils to split excitation so as to control the section, the phase of supply current is required to be fixed for section switching.
Since only the restraint between superconductors and magnets is used as guidance force during running, sufficient guidance force cannot be obtained. When superconductors which generate strong restraint or strong flux trapping force are used, the guidance force increases, though it acts as braking force for driving, which is a disadvantage to the device.
For the above reason, the inductive linear motor of (6) is in principle suitable for a levitation device using the diamagnetism of superconductors.